Thermal management system with variable-volume material

ABSTRACT

A thermal management system configured to be installed between a heat source and a heat sink, including a first heat conductor and a second heat conductor, a thermal switch configured to allow or prevent thermal connection between the first and second heat conductors, the thermal switch including at least one thermally conductive material that can connect the first and second conductors by a change in its volume, and the thermal switch including a controller configured to transfer thermal energy to the phase-change material to change a connection state. The connection is made when the heat source goes above a critical temperature, since the connection enables a heat flux to be established between the heat source and the heat sink.

TECHNICAL FIELD AND PRIOR ART

The present invention relates to a thermal management system for controlling the transfer of a heat flux from a heat source to a heat sink.

In the electronics field, rapid changes, relating in particular to increased power densities and operating speeds, are making thermal management through dissipation a dimensioning parameter in the design of electronics systems. Thermal stress has a direct influence on performance, reliability and the cost of an electronic system.

In addition to small-signal transistors and integrated circuits, electronic systems also now contain medium-power transistors in units of the CMS (Surface-Mounted Components) type, the power dissipations of which for an isolated transistor are approaching one watt. In terms of heat fluxes, a thyristor may generate a flux of the order of 100 to 200 W/cm². Some power electronics components for military applications may generate a heat flux of the order of 300 W/cm² and systems using laser diodes up to 500 W/cm². Non-uniform distribution of these heat fluxes in a printed circuit board may result in the presence of areas having a heat flux of up to five times higher than the average heat flux found in a printed circuit board (˜30 W/cm²).

A recent study by the US Air Force concluded that nearly 55% of failures of electronic systems are due to heat problems.

Management and dissipation of heat are therefore crucial problems for maintaining the temperature of each element at its nominal operating temperature, which varies depending on the field of application.

Several techniques exist to dissipate heat from electronics systems.

Passive radiators exist which consist of fins made of thermally conductive material which transfer the heat emitted by the active components to the ambient air.

Active radiators also exist; these are radiators fitted with ventilators, the ventilator being positioned above the radiator to facilitate heat extraction.

Ventilators are also commonly used, for example, for the whole of the casing of the electronic device. The air is then subject to forced convection, substantially increasing the heat exchange coefficients.

Internal circulation of a fluid, for example by means of a pump, may also be envisaged. Phase-change materials have also been envisaged for passive cooling of electrical and electronic components. Heat pipes and micro heat pipes are also effective. Microchannels have also been envisaged to extract the heat from electronic components. However, a pump is used which may cause disruption and noise disturbance.

Thermal management in the electronics field is therefore particularly critical, especially since systems are providing an increasing number of functions within an increasingly small volume.

DESCRIPTION OF THE INVENTION

The aim of the present invention is to provide a simple and robust system for thermal management of the operating heat flux, which is able to guide the heat flux, for example with a view to its dissipation.

The aim stated above is attained through a thermal management system comprising means able to put two heat conductors in thermal communication, or to prevent such communication these means comprising a material the volume of which varies according to temperature changes. The variable-volume material may therefore be heated for example either by one of the conductors or by an additional heat conductor forming thermal control means.

Management of the heat transfer between the two conductors can then be accomplished automatically and very safely.

In one embodiment, the thermal management system according to the invention may then form a thermal switch made of a volume-expansion material, this switch allowing, in its closed state, heat conduction between the two conductors. The heat flux transmitted from one conductor to the other may vary gradually. In another embodiment the heat system may “manage” the thermal conduction between 2 conductors by a heat flux deriving from a third conductor; in this case it may be considered equivalent to a “thermal transistor”. In one variant, the volume-expansion material may have other physical properties such as, for example, electrical conduction, the management system then having a dual function, both thermal and electrical.

For example, the material used is a solid-liquid phase-change material.

The subject-matter of the present invention is then a thermal management system intended to be installed between at least one heat source and at least one heat sink, comprising a first and second heat conductor, a thermal switch able to allow or prevent thermal connection between the first and second heat conductors, said thermal switch comprising at least one thermally conductive material, the volume of which varies according to a thermal energy input, said material being able to connect the first and second conductor by means of a change in its volume, and said thermal switch comprising control means able to provide thermal energy to the variable-volume material to change the connection state.

The thermal management system may comprise at least three heat conductors, the switch being able to put the at least three heat conductors into thermal connection with one another.

In one example embodiment the thermal management system may be distributed in several planes.

In one embodiment the control means are formed by the first conductor, said first conductor being in permanent contact with the variable-volume material, and is intended to be connected to the at least one heat source.

In another embodiment, the control means are formed by an additional heat conductor, the first and second conductors being not in contact with the variable-volume material in an unconnected state.

In another embodiment the control means are formed directly by the external environment, where the heat energy is provided to the material by convection.

For example, the first and second conductors have ends intended to be in contact with the variable-volume material, said end of the first and/or second conductor being shaped so as to provide gradual contact between said end and the variable-volume material when the volume of the variable-volume material changes.

The thermal management system preferably comprises means to facilitate the variable-volume material's return to a predetermined area in which the first and second conductors are thermally unconnected, called the disconnection area.

The thermal management system may comprise a substrate in or on which the heat conductors are formed, and in which the switch comprises a cavity formed in the substrate and containing the variable-volume material, the heat conductors penetrating into said cavity.

For example, the cavity comprises at least one inclined side wall facilitating the return of the variable-volume material into an area called the disconnection area, said at least one inclined surface forming the means facilitating the return of the variable-volume material to the disconnection area.

The cavity may have a flared shape, for example the shape of an inverted pyramid.

In one example embodiment the two heat conductors are formed on the surface of the substrate, the end of the first conductor penetrating into the cavity more deeply than does the end of the second conductor (and in which the variable-volume material is in contact only with the end of the first conductor in a unconnected state.

In another example embodiment the first conductor is located in the substrate and its end emerges in a base of the cavity, and the second conductor is formed on the substrate.

The cavity may be closed in sealed fashion by a cover.

In a preferred example the variable-volume material is a material which is subject to a phase change. The material may have a solid-liquid phase change in the temperature range which is to be managed by the system. The material subject to a phase change preferably comprises particles improving its thermal conductivity.

The variable-volume material may be a monophasic material, for example a liquid material, for example mercury.

In one variant the variable-volume material is functionalised such that it has a given electrical conductivity, sensitivity to magnetic fields, or is subject to a photoluminescence phenomenon.

The substrate may be made from a material of low thermal conductivity. For example, it may be a thermally insulating material such as a polymer, glass or a ceramic.

Alternatively, the substrate may be a surface-insulated thermally conducting material such as, for example, a silicon substrate with an oxide layer on its face, on which the first and second heat conductors are formed.

For, the heat conductors are metallic, such as for example gold, copper or aluminium.

Another object of the present invention is a thermal management assembly comprising at least two systems according to the present invention, in which the switch of one of the systems is controlled by the first system.

Control may be provided by the temperature of the variable-volume material of the switch of the first system, or that of the heat source or that of the heat sink.

The variable-volume material of the first system may be different to that of the second system.

Another subject-matter of the present invention is an electronic device comprising at least one thermal management system according to the present invention and/or at least one thermal management assembly according to the present invention.

The heat source is, for example, formed by at least one electronic component, and the at least one heat sink is, for example, formed by a heat exchanger, for example a radiator and/or an air flow, and/or at least one micro heat pipe and/or a phase-change material and/or microchannels and/or means implementing convective boiling and/or thermoelectric materials.

BRIEF DESCRIPTION OF THE ILLUSTRATIONS

The present invention will be better understood using the description which follows and the illustrations, in which:

FIG. 1A is a schematic representation of an embodiment of a thermal management system according to the present invention in a thermally unconnected state,

FIG. 1B is a schematic representation of the thermal management system of FIG. 1A in a thermally connected state,

FIG. 2A is a schematic representation of a thermal management system forming a gradual thermal connection,

FIGS. 2B to 2E represent schematically variant embodiments of systems providing a variable heat transfer,

FIG. 3A is a schematic representation of an embodiment of a thermal management system comprising separate control means, in a thermally unconnected state,

FIG. 3B is a schematic representation of the thermal management system of FIG. 3A in a thermally connected state,

FIG. 4A is a schematic representation of an embodiment of a system for thermal management by the external environment, in a thermally unconnected state,

FIG. 4B is a schematic representation of the thermal management system of FIG. 4A in a thermally connected state,

FIGS. 5A and 5B are schematic representations of embodiments of the thermal management system according to the invention,

FIGS. 6A and 6B are schematic representations of a thermal system in several planes in a thermally unconnected state, and in a thermally connected state, respectively,

FIG. 7 is a schematic representation of an architecture comprising several interconnected thermal management systems,

FIG. 8A is a perspective view of a practical example embodiment of a thermal management system according to the invention,

FIG. 8B is a section of the system of FIG. 8A comprising the thermally conductive fluid,

FIG. 8C is a view similar to that of FIG. 8B fitted with encapsulation means,

FIG. 9 is a graphical representation of the variation in height in μm of a phase-change material, and of the temperature difference in ° C. applied according to the % volume increase of the phase-change material in the system of FIG. 8A,

FIG. 10 is a perspective view of a variant embodiment of the system of FIG. 8A.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

In all the figures the heat fluxes are represented symbolically by arrows in the form of chevrons.

In FIGS. 1A and 1B a first embodiment of a thermal management system according to the invention may be seen comprising a first heat conductor 2 and a second heat conductor 4 able to be connected by a thermal switch 6. First conductor 2 is connected by a first end 2.1 to a heat source SC and by a second end 2.2 to switch 6, and second heat conductor 4 is connected by a first end 4.1 to switch 6 and by a second end 4.2 to a heat sink SF.

Heat source SC is, for example, formed by a power transistor, a laser diode, an integrated circuit, etc., and a heat sink SF is, for example, formed by a passive radiator, an active radiator, etc.

The system is produced in a substrate 8.

Heat conductors 2, 4 are, for example, produced in the form of metal tracks on the surface of substrate 8.

As a variant, the conductors may be formed by wires, and may be buried.

The substrate preferably has a low thermal conductivity, typically less than 5 W/° K·m, being for example made of glass or polymer. As a variant, it may also be envisaged to insulate conductors 2, 4 thermally relative to the substrate, by producing a barrier layer between the conductors and the substrate. For example, in the case of a silicon substrate, the barrier layer may be generated by surface oxidation, in order to obtain a layer of silicon dioxide.

Switch 6 comprises a chamber 10 containing a material 12, the volume of which increases when it is heated. Above a given temperature, called critical temperature T_(C), the material provides a thermal connection between the first and second conductors. The heat-transfer material partially fills chamber 10, at least when it does not thermally connect the two heat conductors.

The variable-volume material will be called the “heat-transfer material” in the remainder of the description. This may be a monophasic material, for example a material which is liquid whatever the operating temperature of the system, such as mercury. In this case its volume increases with the temperature and becomes sufficient, when the temperature reaches T_(C), to be in contact with both conductors.

It may be a diphasic material which is in the solid state below the critical temperature, and liquid above critical temperature T_(C), for example a phase-change material. In this case the volume of this material in the liquid state is greater than that in the solid state. When the temperature reaches T_(C) the material becomes molten and its volume increases; the thermal connection between the conductors is then made when the volume of the material is sufficient. These may be materials which are generally designated “phase-change materials”, but any materials with a solid-liquid transition may also be used.

Second end 2.2 of first conductor 2 penetrates into chamber 10, and first end 4.1 of second conductor 4 also penetrates into chamber 10, such that conductors 2, 4 can come into contact with heat-transfer material 12, and effectively be thermally connected to one another through the latter above the critical temperature.

Examples of materials which may be used will be described in detail below.

Switch 6 preferably comprises means to ensure that the heat-transfer material is once again only in contact with the second end of the first conductor when the temperature descends below the critical temperature. These means will be described below.

In FIG. 1B, in which neither of heat conductors 2, 4 are connected, heat-transfer material is in contact with first conductor 2 connected to heat source SC; heat-transfer material 12 is thus connected to heat source SC and experiences all temperature variations of heat source SC. When the temperature of heat source SC goes above critical temperature T_(C), the volume of heat-transfer material 12 is sufficient to come into contact with end 4.1 of second conductor 4. The thermal connection between the two conductors 2, 4 is then made (FIG. 1B), and the heat flux is transferred from heat source SC to heat sink SF. The heat is then dissipated, and heat source SC is cooled. The volume variation may, for example, be of the order of 10%. The required volume variation depends on the properties of the variable-volume material. The size of the chamber and the dimensions of the conductors are chosen accordingly.

Conversely, when the temperature of heat source DC drops and becomes less than critical temperature T_(C), the volume of heat-transfer material 12 is reduced sufficiently that it is no longer in contact with second conductor 4. The latter regains its initial position of being in contact only with first conductor 2, which interrupts the thermal connection between the two conductors 2, 4.

Switch 6 is therefore directly controlled by the heat emitted by heat source SC.

In FIG. 2A a schematic representation is shown of the different steps of the variation of volume of the heat-transfer material, these steps being represented as dashed lines 12.1. FIG. 2A shows the gradual increase of the volume of heat-transfer material. The covering of first end 4.1 of second conductor 2 by heat-transfer material 12 is therefore gradual. And since the heat transfer is directly proportional to the contact area, the transmitted heat flux therefore also increases gradually. Switch 6 consequently forms a thermal transfer variation. This gradualness of the transfer depends in particular on the dimension of first end 4.1 of second conductor 4 penetrating into chamber 10, and which may therefore be surrounded by heat-transfer material 12. Indeed, the shorter this length, the more rapidly the first end is fully covered. The gradualness of the thermal connection may thus be adjusted. This gradualness may allow a certain degree of regulation within the system.

The geometry and volume of the chamber and the containment in which it results, the quantity of variable-volume material, the distance between the conductors, the choice of material and the surface conditions to define the wettability of the surface and to define the migration process of the liquid front enable the reactivity of the switch to be adjusted. For example, by preferring a small chamber volume, a short distance between the conductors, a wetting surface condition to facilitate thermal transfer and a rapid phase transition of the phase-change material, a reactive device may be obtained.

In FIGS. 2B to 2E various example embodiments of a first end 4.1 of second conductor 4 may be seen enabling the gradualness of the thermal connection to be improved.

In FIG. 2B first end 4.1 of second conductor 4 comprises a face 14 which is skew relative to the front of heat-transfer material 12. The contact between heat-transfer material 12 and first end 4.1 thus occurs gradually, as may be seen in FIG. 2C.

In FIG. 2D, in addition to the skew face, end 4.1 of conductor 4 is fitted with fingers 16; by this means the contact between heat-transfer material and first end 4.1 is even more gradual. Heat-transfer material 12 first comes into contact with longest finger 16.1, and then with second finger 16.2; simultaneously it covers finger 16.1 to a greater extent; and it continues in this fashion until it covers all fingers 16, and attains a maximum thermal connection area.

In the example represented in FIGS. 2B to 2E, the first end of the second conductor has a larger section than the remainder of the conductor, but this is in no sense restrictive.

In FIGS. 3A and 3B another embodiment of a thermal management system may be seen in which the switch is controlled by a source of heat other than the heat source.

The system comprises a third heat conductor 18, one end of which penetrates into chamber 10, and another end of which is connected to a source of heat. In addition, when the temperature is less than the critical temperature, heat-transfer material 12 is in contact only with third conductor 18. In the represented example, third conductor 18 is positioned between the two ends 2.2, 4.1 of first conductor 2 and second conductor 4.

When critical temperature T_(C) is applied to the third conductor, the volume of heat-transfer material 12 is sufficient to bring ends 2.2, 4.1 of first conductor 2 and second conductor 4 into contact, thus providing the thermal connection between the two conductors 2, 4. When the temperature of third conductor 18 is less than critical temperature T_(C), the volume of heat-transfer material 12 is such that it is no longer in contact with either of the first and second conductors. It returns to its initial position, and the thermal connection is interrupted.

In this case the change in volume of the heat-transfer material occurs in two opposite directions.

The temperature of third conductor 18 may be imposed by another thermal management system, as will be seen below.

As a variant, the third conductor may be positioned opposite the two heat conductors.

In FIGS. 4A and 4B, another embodiment may be seen in which the thermal connection is controlled by the overall temperature of the device in which the system is installed. Chamber 10 is subject to the external temperature imposed by the entire device which it is desired to cool, and the thermal exchanges occur by natural convection through the wall of chamber 10. When the temperature is below a critical temperature T_(C) the heat-transfer material is contained between the ends of the two conductors 2, 4 and there is no contact with them. When this temperature goes above critical temperature T_(C), the volume of heat-transfer material 12 is sufficient to connect the two conductors 2, 4 thermally. In this example the material of the wall of chamber 10 is such that it facilitates natural convection.

In FIGS. 5A and 5B variant embodiments of the thermal switch according to the invention may be seen. In FIG. 5A the switch enables four conductors 2, 4, 4′, 4″ to be connected; in this case the connection is controlled by the heat source as in the system of FIG. 1A.

In FIG. 5B the switch enables three heat conductors 2, 4, 4′ to be connected; connection is controlled by a conductor 18 which is independent of heat source SC as in the example of FIG. 5B.

In FIGS. 6A and 6B an example of a thermal management system in several planes may be seen.

The system comprises a first system similar to that of FIG. 1A comprising a heat source SC, a first conductor 402 connected to heat source SC, a second conductor 404 connected to a first heat sink SF1 and a switch 406 intended to connect the two conductors 402, 404, on which is superposed a substrate 408 containing a heat conductor 422 traversing substrate 408 forming a thermal via, a conductor 424 deposited on substrate 408 and a second heat sink SF2. Thermal via 422 emerging above switch 406 in the upper wall contains an aperture to allow the heat-transfer material to be brought into contact with thermal via 422.

Switch 406 is directly controlled by heat source SC. When the temperature goes above critical temperature T_(C), the volume of the heat-transfer material makes the thermal connection between first conductor 402 and second conductor 404, and thus heat source SC with first heat sink SF1. When the heat accumulated in the heat-transfer material increases, its volume increases until it reaches the upper wall of switch 406 and comes into contact with thermal via 422. Heat source SC is then connected to first heat sink SF1 and to second heat sink SF2.

The volume variation occurs in two orthogonal directions.

In this example embodiment, if variable-volume material 12 is a solid-liquid phase-change material, the thermal connection between heat source SC and first heat sink SF1 is made when only a portion of material 12 has changed to the liquid state. The connection with second heat source SC2 is made after an additional portion of the material has changed to the liquid state.

This system advantageously enables cooling safety to be improved. If the heat flux between the heat source and the first heat sink proves to be insufficient to lower the temperature of the heat source below the critical temperature, switch 406 detects this, and provides a connection with a second heat sink, in order to increase the heat flux extracted from the heat source.

It may be envisaged that one or more heat sources are connected to several heat sinks by means of systems according to the present invention.

In FIG. 7 an example architecture of an assembly of interconnected management systems according to the intervention may be seen.

The architecture of FIG. 7 comprises a system S1 which is similar to that of FIG. 1A, a management system S2 similar to that of FIG. 3A, and a third management system S3 combining management systems of FIGS. 1A and 3A.

First system S1 comprises a first conductor 102 connected to a first heat source SC1, a second conductor 104 connected to a first heat sink SF1, and a switch 106 directly controlled by the temperature of heat source SC1.

Second system S2 comprises a first conductor 202 connected to a second heat source SC2, a second conductor 204 connected to a second heat sink SF2, and a switch 206. A third conductor 218 controls the switching of second conductor 206, where third conductor 218 is at the temperature of first switch 106 and therefore at the critical temperature of first system S1.

Third system S3 comprises a first conductor 302 connected to a third heat source SC3 and a second conductor 304 connected to a third heat sink SF3 through switch 306′, where conductor 302 and second conductor 304 are able to be connected by a third switch 306 directly controlled by the temperature of heat source SC3. Third system S3 also comprises a fourth switch 306′ controlled by a conductor 318 connected to second heat sink SF2. The third system also comprises a conductor 320 connecting fourth switch 306′ to third heat sink SF3.

The switches have different critical temperatures such that:

T _(C106) <T _(C206) <T _(C306) <T _(C306)′.

These variations of critical temperature may be obtained either by altering the design of the device, or by altering the composition and nature of the material.

We shall now explain the operation of this assembly.

When first heat source SC1 reaches critical temperature T_(C), the heat-transfer material of first switch 206 brings into contact first and second conductors 102, 104, and the thermal connection between the first heat source and first heat sink is made.

Third conductor 218 of second system S2 is covered by the heat-transfer material of first switch 106 of first system S1; the latter is then at critical temperature T_(C). The volume of the heat-transfer material of second switch 206 increases, making the thermal connection between second heat source SC2 and heat sink SF2. In the represented configuration the material of first switch 106 will reach third conductor 218 before reaching second conductor 104.

As regards third system S3, the thermal connection between third heat source SC3 and third heat sink SF3 is controlled by switch 306, which is directly controlled by third heat source SC3 and switch 306′, which is controlled by second heat sink SF2. Third heat source SC3 will initially be in contact with second heat sink SF2, and then in contact with second and third heat sinks SF2 and SF3, if the heat flux is sufficient.

It should be noted that in order to activate a switch the substantial heat before the change of phase and then the latent heat causing the change of phase must first be accumulated. The material then has a buffer effect in the heat transfer. This buffer effect means that this substantial heat absorption before the change of phase is able to allow very short temperature rises, requiring no connection to the heat sink, to be absorbed. In the case of a high and persistent temperature the connection is indeed made after absorption of the phase-change heat.

By virtue of the invention, architectures may thus be produced with a number of inputs and outputs which are adjustable and configurable in terms of dimension, material and shape.

Each of the systems may comprise a heat-transfer material which is different to that of the other systems, and which is suitable, for example, for the critical temperature of each system.

We shall now describe a practical example embodiment of a thermal management system according to the present invention.

In FIGS. 8A to 8C, a perspective view of an example embodiment of a thermal management system according to the present system such as that of FIG. 1A may be seen. A cavity 26 forming chamber 10 of switch 6 is made in substrate 8, and first conductor 2 and second conductor 4 are produced in upper face 8.1 of substrate 8.

First conductor 2 comprises one end 2.2 penetrating into cavity 26 more deeply than end 4.1 of second conductor 4, such that first conductor 2 is in contact with the heat-transfer material when this material is not expanded.

Cavity 26 is shaped such that it facilitates the return of the heat-transfer material to its unconnected position. Cavity 26 has a deeper first portion 26.1 in the shape of a parallelepipedic rectangle, and a second portion 26.2 emerging at the surface of substrate 8; the second portion has two inclined opposite faces 28 opening towards the upper surface.

The surfaces of the cavity may advantageously be subjected to a treatment facilitating “dewetting” of the material when the volume of the material is reduced, allowing its return to a state of disconnection of the system to be improved.

The parallelepipedic shape of the base of the cavity is in no sense restrictive, and it could have an inclined shape, cylindrical shape, rounded shape, etc.

First conductor 2 extends along the entire length of side 28 of second portion 26.2, while second conductor 4 extends only over a portion of a side 28 opposite that on which first conductor 2 is located.

The heat-transfer material fills first portion 26.1 and the base of second portion 26.2 such that it is in contact with first conductor 2, as may be seen in FIG. 1C.

The switch is directly controlled by the heat source. When the temperature goes above critical temperature T_(C), the heat-transfer material comes into contact with the second conductor, and provides the thermal connection.

When the temperature is again below critical temperature T_(C), the volume of the heat-transfer material has decreased such that it is no longer in contact with second conductor 4. The flared shape of second portion 26.1 of cavity 26 ensures that the heat-transfer material returns to its unconnected position.

It could be arranged for the cavity to have inclined walls over its entire height.

In addition, cavity 26 could have a tapered shape. The shape of FIG. 8A has the advantage that flat conductors can be produced on the sides of the second portion of the cavity.

In FIG. 10 another example embodiment of a switch may be seen in which the second portion of the cavity has an inverted pyramid shape. In this example, first conductor 2 is produced within substrate 8 and emerges in the base of cavity 26, and second conductor 4 is similar to that of FIG. 8A. However, it extends more deeply than second conductor 4 of FIG. 8A, since the quantity of heat-transfer material may be smaller, and the contact with first conductor 2 occurs in the base of cavity 26.

Cavity 26 preferably comprises at least one inclined side.

The system may also being encapsulated, as is represented in FIG. 8C. The system of FIG. 8C has a cover 30 closing cavity 26 in sealed fashion, thereby making the device very practical and easy to use. Cover 30, made for example of glass or metal, is for example attached in sealed fashion on to substrate 8 by a bead of cement.

We shall explain the operation of the switch of FIG. 8A. We consider the case in which a diphasic phase-change (solid/liquid) material is chosen as the heat-transfer material.

In the example represented in FIG. 8A, the surface of the phase-change material is in contact with the ambient air, with which it exchanges heat only by natural convection.

When the first conductor is at a temperature above critical temperature T_(C) the phase-change material begins to melt. This melting absorbs energy in latent form. Natural convection currents gradually occur in the liquid phase-change material. To remedy the low thermal conductivity of the phase-change materials, we can consider a phase-change material formed from a pure paraffin wax and nanoparticles of highly conductive graphite. Certain compositions bring the thermal conductivity of the phase-change material to a value of around one (SI) [0]. After the start of the natural convection in the liquid phase-change material the melting process is driven by natural convection.

When it changes to the liquid phase the phase-change material increases in volume and comes into contact with second conductor 4.

As an example, the change in height of the surface of the phase-change material and also the increase of volume of the phase-change material, required for making the thermal connection, may be calculated.

For example, considering a phase-change material filled with graphite nanoparticles the characteristics of which are:

-   -   Dynamic viscosity (μ): 5×10⁻³ Pa·s,     -   Density (ρ): 800 kg·m⁻³,     -   Thermal conductivity (λ): 1 W·m⁻¹ K⁻¹,     -   Specific heat (cp): 2500 J·kg⁻¹ K⁻¹,     -   Latent heat (L): 200 kJ·kg⁻¹,     -   Coefficient of thermal expansion (β): 10-3 K⁻¹,     -   Prandtl number: 12.5,     -   Phase-change temperature (T_(pc)): 20° C.

On this basis, the variation of height of the surface of the phase-change material (h) may be calculated as a function of the increase of the volume of the phase-change material (expressed as a %). The correspondence between the increase of the phase-change material's volume and the temperature difference applied to the material (ΔT) may also be established by the following relationship:

ΔV=β×V ₀ ×ΔT

where ΔV is the increase in volume and V₀ is the initial volume.

In FIG. 8C the variation of height of heat-transfer material in cavity 26 can be seen represented symbolically.

In FIG. 9 the variation in height (in μm) of the phase-change material and in temperature difference (in ° C.) applied may be seen as a function of the increase of volume (in %) of the phase-change material.

These curves indicate, for example, that the surface of the phase-change material will reach second conductor 4 connected to the heat sink when its volume has increased by approximately 6.20, the height then being h_(c)=2100 μm. Bearing in mind that the material has a coefficient of thermal expansion equal to 10⁻³ K⁻¹, this increase of volume corresponds to a temperature difference of approximately 62° C.

As described above, the heat-transfer material is, for example, a phase-change material. Phase-change materials have the advantage that they are available in wide temperature ranges.

Phase-change materials have the characteristic that they can store energy in the form of latent heat. The heat is absorbed or emitted during the change from the solid state to the liquid state, and vice versa.

A phase-change material having a solid-liquid transition in the temperature range in question is preferably chosen. Phase-change materials having a solid-liquid transition in the temperature range considered may be envisaged, however the transformation is generally slow, which may be disadvantages for the reactivity of the system.

Above a certain temperature characteristic of each phase-change material, phase-change materials liquefy by absorbing heat from the ambient atmosphere, and emit it when the temperature drops.

This property is related to its substantial melt energy per unit of volume; the higher this is the more advantageous will be the heat storage/emission properties.

Paraffins may for example be chosen, such as eicosane, docosane and tricosane, or other inorganic materials such as hydrates of salts or metal hydrides. Paraffins have the advantage that they are thermally stable and inexpensive. Conversely, they have relatively low thermal conduction. Paraffins may also be used in association with heat conductive elements capable of transferring heat within and outside the material efficiently. These elements may be thermal dissipators, partitions, fins, graphite nanofibres, metal foams, dispersed conductive particles, micro-encapsulations of phase-change material, or carbon nanotubes the presumed thermal conductivity of which is very high.

As regards substrate 8, as mentioned above, it preferably has a low thermal conductivity in order to limit heat loss from thermal conductors 2, 4. The substrate may be made of a polymer material such as epoxy, or of a ceramic. Silicon may also be envisaged, which is used for the manufacture of electronic components. Bearing in mind its relatively high thermal conductivity, a thermal barrier layer, for example an SiO2 oxide layer, will be formed on the upper face of the substrate where the conductors are positioned.

Epoxy has a thermal conductivity of 0.25 W·m⁻¹·K⁻¹. Ceramics have a thermal conductivity of the order of 0.49 W·m⁻¹·K⁻¹. Silicon has a thermal conductivity of 149 W·m⁻¹·K⁻¹ and silicon oxide has a thermal conductivity of several Watt·m⁻¹·K⁻¹.

The substrate is structured using techniques widely known to those skilled in the art: casting, machining; as regards silicon, microelectronic techniques are used: wet etching, dry etching, electrochemical etching, etc.

As regards the production of the conductors, they may be produced by photolithography.

The conductors are, for example, made of aluminium, of thermal conductivity equal to 237 W·m⁻¹·K⁻¹, of gold, of thermal conductivity equal to 317 W·m⁻¹·K⁻¹, or of copper, of thermal conductivity equal to 390 W·m⁻¹·K⁻¹.

It may be envisaged to functionalise the heat-transfer material such that it has a certain electrical conductivity, sensitivity to magnetic fields, or is subject to a photoluminescence phenomenon. This functionalisation may be obtained, for example, through the addition of nanomaterials.

The system may then form an electrical and/or optical and/or magnetic switch.

The thermal management system according to the invention forms a thermal switch determining whether or not a heat flux flows. Switching may occur automatically, and provide thermal regulation between the heat source and the heat sink in an autonomous manner.

It may also form a thermal regulator, since the gradual change in volume of the heat-transfer material enables the heat flux transfers to be regulated.

By combining several systems according to the invention, heat fluxes may be ordered, directed and distributed. In addition, variable-volume heat-transfer elements store heat; they therefore form a buffer to the advance of the heat flux. By this means they form heat sinks. 

1-28. (canceled)
 29. A thermal management system configured to be installed between at least one heat source and at least one heat sink, comprising: a first heat conductor and a second heat conductor; a thermal switch configured to allow or prevent thermal connection between the first and second heat conductors, the thermal switch including at least one thermally conductive material, a volume of which varies according to a thermal energy input, the material configured to connect the first and second conductor by a change in its volume, and the thermal switch including a controller configured to provide thermal energy to the material to change a connection state.
 30. A thermal management system according to claim 29, comprising at least three heat conductors, the switch being configured to put the at least three heat conductors into thermal connection with one another.
 31. A thermal management system according to claim 29, distributed in plural planes.
 32. A thermal management system according to claim 29, in which the controller is formed by the first conductor, the first conductor being in permanent contact with the material, and is configured to be connected to the at least one heat source.
 33. A thermal management system according to claim 29, in which the controller includes an additional heat conductor, the first and the second conductors not being in contact with the material in an unconnected state.
 34. A thermal management system according to claim 29, in which the controller is formed directly by an external environment, and the thermal energy is provided to the material by convection.
 35. A thermal management system according to claim 29, in which the first and second conductors have ends configured to be in contact with the material, the end of the first and/or second conductor being shaped to provide gradual contact between the end and the material when the volume of the material changes.
 36. A thermal management system according to claim 29, configured to facilitate return of the material to a predetermined area in which the first and second conductors are thermally unconnected in a disconnection area.
 37. A thermal management system according to claim 29, further comprising a substrate in or on which the heat conductors are formed, and in which the switch comprises a cavity formed in the substrate and containing the material, the heat conductors penetrating into the cavity.
 38. A thermal management system according to claim 37, in which the cavity comprises at least one inclined side wall facilitating return of the material into a disconnection area.
 39. A thermal management system according to claim 37, in which the cavity has a flared shape.
 40. A thermal management system according to claim 37, in which the two heat conductors are formed on the surface of the substrate, the end of the first conductor penetrating into the cavity more deeply than the end of the second conductor, and in which the material is in contact only with the end of the first conductor in an unconnected state.
 41. A thermal management system according to claim 37, in which the first conductor is formed in the substrate and its end emerges in a base of the cavity and the second conductor is formed on the substrate.
 42. A thermal management system according to claim 37, in which the cavity is closed in a sealed fashion by a cover.
 43. A thermal management system according to claim 29, in which the variable volume material is a phase change material.
 44. A thermal management system according to claim 43, in which the material has a solid-liquid phase change in a temperature range which is to be managed by the system.
 45. A thermal management system according to claim 43, in which the material subject to a phase change comprises particles increasing its thermal conductivity.
 46. A thermal management system according to claim 29, in which the variable volume material is a monophasic material.
 47. A thermal management system according to claim 29, in which the variable-volume material is functionalized such that it has a given electrical conductivity, sensitivity to magnetic fields, or is subject to a photoluminescence phenomenon.
 48. A thermal management system according to claim 29, in which the substrate is a material of low thermal conductivity.
 49. A thermal management system according to claim 29, in which the substrate is a thermally insulating material.
 50. A thermal management system according to claim 29, in which the substrate is surface-insulated thermally conducting.
 51. A thermal management system according to claim 29, in which the heat conductors are metal.
 52. A thermal management assembly comprising at least two systems according to claim 29, in which the switch of one of the systems is controlled by the first system.
 53. A thermal management assembly according to claim 52, in which control is provided by a temperature of the material of the switch of the first system, or that of a heat source or that of a heat sink.
 54. A thermal management assembly according to claim 52, in which the material of the first system is different than that of the second system.
 55. An electronic device comprising at least one thermal management system according to claim 29 and/or at least one thermal management assembly.
 56. An electronic device according to claim 55, in which the heat source is formed by at least one electronic component, and at least one heat sink is formed by a heat exchanger.
 57. A thermal management system according to claim 29, in which the material is a liquid monophasic material.
 58. A thermal management system according to claim 50, in which the substrate is a silicon substrate with an oxide layer on its face, on which the first and second heat conductors are formed. 